Bumper beam for motor vehicles

ABSTRACT

A bumper beam for motor vehicles includes vertical front and rear plates made of at least one metallic material. At least one elongated core made of metallic material is positioned between the two sole plates. An energy absorber is located at each end of the core and formed of a metallic hollow body, the energy absorber being positioned at right angles to the sole plates or to the core. The metallic materials of the core and the energy absorber have a ratio of elastic limit to breaking stress that is lower than that of the sole plate metallic material and lower than 0.9.

FIELD OF THE INVENTION

The present invention relates to a bumper beam for motor vehicles.

BACKGROUND OF THE INVENTION

Motor manufacturers are concerned with improving passenger safety andwith reducing the cost of repairing vehicles in the event of impacts,while at the same time avoiding any significant increase in the weightof these vehicles.

For safety particularly in the event of a frontal impact, manufacturersare optimizing the various parts of the vehicle so as to guaranteemaximum absorption of kinetic energy by mechanical deformation of zonesaway from the cabin and so as to strengthen the cabin in order toprotect the passengers.

For repairing vehicle parts damaged after a frontal impact,manufacturers are looking first of all to raise the speed at whichirreversible damage occurs to as high as possible a speed, this speedbeing of the order of 5 km/h, and then to limit the damage to the endsof the vehicle, that is to say to zones which can easily be repaired, upto speeds of the order of 15 km/h.

The safety objective is generally achieved by providing the vehicle withlongerons, the gradual deformation of the ends of which absorbs thekinetic energy of the vehicle, the zones of the longerons locatedclosest to the cabin and the cabin itself being designed to deform verylittle.

However, given the limited space in motor vehicles, manufacturers, arehaving to develop structures which guarantee a good level of energyabsorption from the end of the vehicle up to the non-deformable zonewhile at the same time avoiding dead zones, that is to say zones whichcannot deform axially and absorb significant deformation work.

Furthermore, manufacturers have reduced the costs of repair in low-speedimpacts by modifying the bumpers or the scuff moldings of the vehicles,particularly by strengthening the bumper beam and inserting, betweenthis beam and the longerons, deformable mechanical elements known asenergy absorbers which are designed to crumple under a force which isappreciably lower than the force required to deform the longerons.

In the event of a low-speed impact, all that is therefore required isfor a limited number of parts to be replaced, thus limiting repaircosts.

Repairs are also improved by strengthening the bumper beam so as toraise the speed of impact without damage and to guarantee gooddistribution of forces in the event of a higher-speed impact.

What happens is that the bumper has to transmit the forces to the energyabsorbers and then to the longerons of the vehicle in the later phasesof the impact.

The way in which this structure, which consists of the bumper beam andof the energy absorbers, works has also to be extremely stable withrespect to angles of impact more or less steeply inclined with respectto the longitudinal axis of the vehicle.

In an attempt to satisfy these criteria, numerous solutions have beenimplemented to date, but they lead to an appreciable increase in theweight of the vehicle and in cost.

One of the known solutions consists in fitting steel blades in bumpersmade of synthetic materials and in incorporating energy absorbersbetween the bumper and the longerons of the vehicle. However, the energyabsorbers used to date are not always satisfactory and entail addingadditional parts, which increases the weight of the vehicle.

To avoid this drawback, energy absorbers are known which are made ofaluminum, which allows the weight to be reduced, but theirenergy-absorption properties are generally not easy to control and ofteninsufficient, while the material costs and cost of manufacture of suchabsorbers remain high.

Also known are energy absorbers made of steel with a relatively complexgeometry which fulfills the safety criteria and allow a slight reductionin weight. However their cost of manufacture also remains very high.

BRIEF DESCRIPTION OF THE INVENTION

An object of the invention is to avoid these drawbacks by proposing abumper beam for motor vehicles which, at the same time, makes itpossible to improve the energy absorption and also to reduce both thecost of repair in the case of a low-speed impact, and the weight of thevehicle.

The subject of the invention is therefore a bumper beam for motorvehicles, characterized in that it comprises a front sole plate and arear sole plate which are vertical and made of at least one metallicmaterial, at least one core made of metallic material and positionedbetween the two sole plates and, at each end of said core, an energyabsorber formed of a hollow body made of metallic material running atright angles to the sole plate and connected to at least said front soleplate or to said core, the metallic materials of this core and of thehollow body of the energy absorbers having a ratio between the elasticlimit and the breaking stress lower than that of the metallic materialof said sole plates and lower than 0.9.

According to other preferred characteristics of the invention:

the metallic material of the sole plates is a steel with a very highelastic limit higher than 400 MPa or an aluminum with a very highelastic limit of above 250 MPa,

the sole plates are made of the same metallic material and havedifferent thicknesses,

the metallic material of the front sole plate has a ratio between theelastic limit and the breaking stress lower than that of the metallicmaterial of the rear sole plate,

the bumper beam has two parallel cores stretching between said soleplates,

the thicknesses of the sole plates and of the cores are different andthe thickness of the sole plates is preferably greater than that of thecores;

said core comprises a succession of alternating projecting parts andrecessed parts running at right angles to the longitudinal axis of thiscore,

the distance between the top of the projecting parts and the bottom ofthe recessed parts of said core is between {fraction (1/20)}th and onehalf of the period of the projecting parts or recessed parts,

the distance between the bottom of the recessed parts of the two coresis between 0 and half the period of said projecting parts or recessedparts,

the hollow body of each energy absorber has a cross section in the shapeof a four-armed cross, the arms extending in twos in the continuation ofone another and making an angle of 90° between them.

Other characteristics and advantages of the invention will becomeapparent in the course of the description which will follow, givenmerely by way of example and made with reference to the appendeddrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a motor vehicle front bumper equipped witha beam according to the invention,

FIG. 2 is a schematic perspective view of the bumper beam according tothe invention,

FIG. 3 is a schematic perspective exploded view of the bumper beamaccording to the invention,

FIG. 4 is a schematic perspective view of the central part of the bumperbeam according to the invention,

FIG. 5 is a schematic view in cross section of an energy absorber of thebumper beam according to the invention,

FIG. 6 is a schematic perspective view of another embodiment of thebumper beam according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically depicts a bumper denoted in its entirety by thereference 1 and which, in the embodiment depicted in this figure, ismounted at the front of a motor vehicle 2 via two parallel longerons 3running at right angles to said bumper 1.

This bumper 1 may equally be mounted at the rear of the motor vehicle.

As depicted in FIGS. 1 and 2, the bumper 1 is formed by a beam 10 whichcomprises a central zone A constituting the crossmember, two lateralzones B each constituting an energy absorber and each arranged one oneither side of the central zone A and two end zones C constituting aprotection for the lateral corners of the motor vehicle 2.

The central zone A of the beam 10 essentially carries out a function ofprotecting the motor vehicle 2 with no visible damage for low-speedimpacts and also a function of spreading and transmitting contact forceto the lateral zones B and then to the longerons 3 for higher-speedimpacts, via said lateral zones B each of which constitutes an energyabsorber, as will be seen later on.

In general, the beam 10 is made from at least two metallic materialswhich differ in terms of their nature, their mechanical properties ortheir thickness.

As depicted in particular in FIGS. 2 and 3, the beam 10 comprises twovertical and longitudinal sole plates, namely a front sole plate 11 aand a rear sole plate 11 b, respectively, made of at least one metallicmaterial which has a very high elastic limit, such as a steel with anelastic limit higher than 400 MPa and preferably between 1000 and 1500MPa or an aluminum with an elastic limit higher than 250 MPa.

The sole plates 11 a and 11 b form between them a gap in which at leastone core 15 a or 15 b is inserted. As a preference, two cores, an uppercore 15 a and a lower core 15 b, respectively, are inserted between thetwo sole plates and are arranged horizontally and parallel to oneanother.

In what follows, the description will be given with respect to anembodiment with two cores 15 a and 15 b running parallel to one another.

The cores 15 a and 15 b are made of a metallic material with highdeformability and preferably of a metallic material having a ratiobetween the elastic limit and the breaking stress lower than that of themetallic material of the sole plates 11 a and 11 b and lower than 0.9.

According to a first embodiment, the sole plates 11 a and 11 b are madeof the same metallic material and have different thicknesses.

According to another embodiment, the metallic material of the front soleplate 11 a has a ratio between the elastic limit and the breaking stresslower than that of the metallic material of the rear sole plate 11 b.

Each core 15 a and 15 b comprises, in the central part A, a successionof alternating projecting parts 16 a and recessed parts 16 b running atright angles to the longitudinal direction of the corresponding core.

In general, the cores 15 a and 15 b have a shape which makes it possibleto reduce their thicknesses while at the same time preserving the theirresistance to buckling, and these cores have a certain ductility and,for preference, a ratio between the elastic limit and the breakingstress lower than 0.9. The projecting parts 16 a and recessed parts 16 bhave a period tailored to optimize the behavior of these cores and ofthe beam.

According to a preferred embodiment depicted more specifically in FIG.4, the period L2 of the projecting parts 16 a and recessed parts 16 b isuniform along the entire length of each core 15 a and 15 b.

In addition, the distance L1 between the axis of a projecting part 16 aand the axis of a recessed part 16 b is equal to half the period L2, andthe distance L3 between the top of the projecting parts 16 a and thebottom of the recessed parts of each core 15 a and 15 b is between{fraction (1/20)}th and one half of the period L1 of said projectingparts or recessed parts.

In addition, the distance L4 between the bottom of the recessed parts 16b of the two cores 15 a and 15 b is between 0 and half the period L1 ofsaid projecting parts or recessed parts.

According to another embodiment, the projecting parts 16 a and recessedparts 16 b of one core 15 a are offset with respect to the projectingparts 16 a and recessed parts 16 b of the other core 15 b so that a lowpoint on one core faces a high point on the other core so that thedistance between the cores 15 a and 15 b is constant.

In the embodiment depicted in the figures, the projecting parts 16 a andrecessed parts 16 b are formed by corrugations. They may equally beformed of ribs and these projecting parts 16 a and recessed parts 16 bare produced by deforming the metal and allow the thickness of the cores15 a and 15 b to be reduced appreciably, for the same buckling andwarping load.

The projecting parts 16 a may equally be formed by depressions or cutswith folded-over edges.

The lateral parts B of the beam 10 arranged on each side of the centralpart A of, said beam 10 each constitute an energy absorber formed by ahollow body 20 made of metallic material and running at right angles tothe sole plates 11 a and 11 b.

The metallic material of which the hollow body 20 of each energyabsorber is made has a ratio between the elastic limit and the breakingstress lower than that of the metallic material of said sole plates 11 aand 11 b and lower than 0.9.

According to a preferred embodiment, depicted in FIGS. 3 and 5, eachhollow body 20 constituting an energy absorber is formed by twosymmetric hollow half-bodies 20 a and 20 b respectively each extendingone core 15 a and 15 b respectively.

The two hollow half-bodies 20 a and 20 b are joined together at theirfree edges 21 a and 21 b respectively, for example by spot welding or bya continuous welding or by crimping or using an appropriate adhesive orby local pressing or alternatively by a seam.

One hollow body 20 of one energy absorber will be described withreference to FIG. 5, the hollow body of the other energy absorber beingidentical.

The hollow body 20 has a cross section in the shape of a four-armedcross, the arms 22, 23, 24 and 25 respectively, extending in twos in thecontinuation of one another and forming an angle of 90° between them.The wall of the hollow body 20 is formed by a succession of facets 26 ofidentical width “l” making an angle α of 135° with each other. Thenumber of facets 26 of the hollow body 20 is equal to twenty-four.

Furthermore, the hollow body 20 of each energy absorber is open on oneof its sides and, in the embodiment depicted in the figures, the hollowbody 20 has an opening 29 arranged on the same side as the central partof the beam 10.

In a variant, the hollow body 20 of each energy absorber is closed onits entire periphery.

In general, the hollow body 20 of each energy absorber is connected toat least the front sole plate 11 a or to the core in the case where thebeam has a single core or to the two cores when the beam comprises twocores, 15 a and 15 b respectively.

The connection between each energy absorber and the front sole plate 11a or the cores 15 a and 15 b can be made in various ways.

According to a first embodiment, the hollow body 20 of each energyabsorber has a first end 27 connected to the front sole plate 11 a forexample by welding, crimping, local pressing or a seam and a second end28 resting on a longeron 3 of the structure of the motor vehicle andpassing freely through the rear sole plate 11 b via an orifice 30 whichhas an outline of a shape that complements the cross section of saidhollow body 20, as depicted in FIG. 3.

According to a second embodiment, the hollow body 20 of each energyabsorber has a first end 27 resting on the front sole plate 11 a and asecond end 28 resting on a longeron 3 of the structure of the motorvehicle. In this embodiment, this second end 28 passes through the rearsole plate 11 b via an orifice 30 which has an outline of a shape thatcomplements the cross section of said hollow body 20 and the hollow body20 is connected to the rear sole plate 11 b for example by welding, bycrimping, by local pressing or by a seam.

According to a third embodiment, the body 20 of each energy absorber hasa first end 27 resting on the front sole plate 11 a and a second end 28connected to the rear sole plate 11 b for example by welding, crimping,local pressing or a seam. In this case, the hollow body 20 of eachenergy absorber does not protrude beyond the rear sole plate 11 b andthis rear sole plate 11 b rests on a longeron 3 of the structure of themotor vehicle.

According to another embodiment, the hollow body 20 of each energyabsorber rests on the front sole plate 11 a and is connected to the core15 a or 15 b or to both cores 15 a and 15 b.

According to yet another embodiment, the hollow body 20 of each energyabsorber is connected to the front sole plate 11 a for example bywelding, by crimping, by local pressing or by a seam and is alsoconnected to the core 15 a or 15 b in the case where the beam 10 hasjust one core or is connected to both cores 15 a and 15 b in the casewhere the beam 10 has two cores 15 a and 15 b.

In general, the hollow body 20 of each energy absorber may be formed ofan independent part fixed to the ends of the core or cores 15 a and 15 bby welding, by crimping or by the rolling of mating edges, or by a seamor by local pressing.

In a variant, the hollow body 20 of each energy absorber can be producedin the form of a closed body formed integrally with the cores 15 a and15 b or fixed to said cores 15 a and 15 b.

The metallic material of the hollow body 20 of each energy absorber hasa thickness preferably of less than 1.2 mm.

Each core 15 a and 15 b has on its longitudinal edge facing the frontsole plate 11 a, a rim 17 for attachment to said front sole plate 11 aand running along the entire length of each core 15 a and 15 b.

Likewise, each core 15 a and 15 b has, on its longitudinal edge facingthe rear sole plate 11 b, a rim 18 for attachment to said rear soleplate 11 b and running only along the length of the central part of eachcore 15 a and 15 b.

The soles 11 a and 11 b can be assembled with the cores 15 a and 15 b byconventional assembly methods of the welding or mechanical type.

In the case of a weld, this weld may be a spot weld or a continuouslaser weld.

However, the lengths of the zones that require connection are great,most particularly in the central zone A, and this significantlypenalizes the cost when performing assembly by welding.

Mechanical assembly techniques are such that the deformation by localpressing allows a great many points to be produced simultaneously, forexample in a press. However, this type of assembly poses problemsbecause of the superior mechanical properties of the metallic materialof which the sole plates 11 a and 11 b are made.

Assembly of the crimping type is achieved by forming, for example, aseam produced by simultaneously rolling the longitudinal edges of thesole plates 11 a and 11 b together with the longitudinal edges 17 and 18of the cores 15 a and 15 b.

This type of assembly can be performed in a single operation andpresents a technical advantage owing to the fact that the cylindricalshape of the rolled zones contributes to the mechanical strengthening ofthe beam 10.

Finally, the outer sole plate 11 a has, at the lateral parts B and atthe end parts C, horizontal rims 31 which cover the body 20 of eachenergy absorber and rest on the edge of the inner sole plate 11 b insaid zones B and C.

FIG. 6 depicts a variant embodiment of the energy absorber.

In this embodiment, each energy absorber consists of a hollow body 20formed of a four-armed cross with arms 22, 23, 24 and 25 respectively.In this case, the cross is inclined so that it forms a “X”. The hollowbody 20 may equally have an opening 29 on the same side as the centralpart A of the beam 10.

The use of a steel with superior mechanical properties for the soleplates 11 a and 11 b, combined with a more ductile steel for the cores15 a and 15 b so as to produce a structure combining the beam andabsorber functions in a single piece, makes it possible to reduce theweight and the cost of producing a bumper comprising such a beam and tooptimize the behavior, particularly by guaranteeing an optimumfunctional travel of the energy absorbers.

The use of a steel with a high elastic limit makes it possible to reducethe weight and combining it with a more ductile steel makes it possibleto produce a single piece of complex shape in spite of the limiteddeformability of high-elastic-limit steels.

The central zone A carries out a function of protecting the vehiclewithout visible damage for low-speed impacts and a function of spreadingand transmitting contact loads to the lateral zones B constituting theenergy absorbers and then to the longerons 3 for higher-speed impacts.

A structure such as this makes it possible to optimize the weight of thebeam 10, because the high-elastic-limit steel of the sole plates 11 aand 11 b allows a high level of strain to be applied before the materialenters the plastic phase and before the onset of permanent deformation.Combining it with a ductile steel for the cores 15 a and 15 b makes itpossible to produce, in these cores, for example by pressing, projectingparts and recessed parts such as corrugations or ribs for example, whichallow the resistance of said cores 15 a and 15 b to warping to beimproved appreciably and thus allow their weight to be reduced for thesame strength.

Assembling the various elements of the beam using seams have advantages.Specifically, it is compatible with steels with very good mechanicalproperties. These steels have a limiting bend radius of several timesthe thickness of the metal, and crimping involves bend radii of theorder of the thickness of metal whereas the radius for assembling byseam can be tailored and chosen. This radius may, for example, be fourto five times the thickness of the high-grade steel.

Compared with the welding method, assembling using a seam also has theadvantage of very high productivity and is also easy to do, in a singlepress operation using a suitable tool.

Assembly by seam presents excellent strength in the plane perpendicularto the axis of the seam. For certain levels of loading and in order toavoid relative slippage along the axis of the seam, that is to saybetween the core and the sole plate, it is possible to insert someadhesive between the two elements at the seam or to weld by, locallyfusing or preferably locally crushing the seam with a press toolcomprising, for example, a V-shaped punch ending in a rounded portionand a flat anvil.

This operation can be performed on a press at a high rate. A suitabletool can simultaneously indent at least two seams and the spacing of theindentations is of the order of five to ten times the outer diameter ofthe seam.

In order to fit in with the geometry of the vehicle, the overall shapeof the sole plates 11 a and 11 b has a slight curvature compatible withthe low shapeability of steels with a high elastic limit. These partsmay be produced, for example, by bending.

The shape of the cores 15 a and 15 b is more complex but is compatiblewith the ability to shape the chosen steel of which to make them.

Thus, this combination of materials therefore makes it possible toproduce a complex shape that meets the esthetic requirements of motorvehicles.

The beam 10 may be coated with a synthetic material 32 (FIG. 1) whichacts as a cushion in low-speed impacts and constitutes the outer skin ofthe bumper, thus giving the part the appearance of the bumpers in commonuse.

The role of the cushion of synthetic material associated with the steelstructure is primordial for low-speed impacts, for example collisionswith a wall or a post.

By deforming elastically, the synthetic material allows the contactpressure to be spread over the beam 10 and allows the force to bereduced in low-speed impacts.

This synthetic material absorbs the kinetic energy of the vehicle byelastic deformation. The synthetic material used can be formed ofsynthetic foams with appropriate mechanical properties, such as cellularrubbers, for example.

The beam according to the invention, intended to form a front or rearbumper for motor vehicles, makes it possible to improve the performanceof this vehicle in the event of a frontal impact and to reduce therepair costs. It has a high level of energy absorption and makes itpossible to reduce the weight and the number of parts by comparison withthe conventional structures.

What is claimed is:
 1. A bumper beam for motor vehicles, comprising:vertical front and rear metallic sole plates; at least one elongatedmetallic core positioned between the sole plates; hollow metallic energyabsorbers formed at both ends of the core, the energy absorberspositioned at right angles to the sole plates or to said core, a ratioof elastic limit to breaking stress for the core and also for the energyabsorbers is lower than the ratio for the sole plates.
 2. The bumperbeam as claimed in claim 1, wherein the metallic material of the soleplates is a steel with a very high elastic limit higher than 400 MPa. 3.The bumper beam as claimed in claim 1, wherein the metallic material ofthe sole plates is an aluminum with a very high elastic limit above 250MPa.
 4. The bumper beam as claimed in claim 1, wherein the sole platesare made of the same metallic material and have different thickness. 5.The bumper beam as claimed in claim 1, wherein the metallic material ofthe front sole plate has a ratio of elastic limit to breaking stressthat is lower than that of the metallic material of the rear sole plate.6. The bumper beam as claimed in claim 1, wherein two parallel coresextend between said sole plates.
 7. The bumper beam as claimed in claim1, wherein the thicknesses of the sole plates and of the cores aredifferent.
 8. The bumper beam as claimed in claim 1, wherein thethicknesses of the sole plates are greater than that of the cores. 9.The bumper beam as claimed in claim 1, wherein said core comprises asuccession of alternating projecting and recessed parts extending atright angles to the longitudinal axis of the core.
 10. The bumper beamas claimed in claim 9, wherein the period of the projecting parts andrecessed parts is uniform along the entire length of a correspondingcore.
 11. The bumper beam as claimed in claim 9, wherein the projectingparts and recessed parts of one core are offset with respect to theprojecting parts and recessed parts of the other core.
 12. The bumperbeam as claimed in claim 9, wherein the distance between the top of theproject parts and the bottom of the recessed parts of said core isbetween {fraction (1/20)}^(th) and one-half of the period of theprojecting parts or recessed parts.
 13. The bumper beam as claimed inclaim 9, wherein the distance between the bottom of the recessed partsof the two cores is between 0 and half the period of said projectingparts or recessed parts.
 14. The bumper beam as claimed in claim 9,wherein the projecting parts are formed by depressions or cuts withfolded-over edges.
 15. The bumper beam as claimed in claim 1, whereinthe hollow body of each energy absorber has a cross section in the shapeof a four-armed cross, the arms extending continuously in twos andmaking an angle of 90° between them.
 16. The bumper beam as claimed inclaim 15, wherein the hollow body of each energy absorber is open on oneof its sides.
 17. The bumper beam as claimed in claim 15, wherein thehollow body of each energy absorber is closed along its entireperiphery.
 18. The bumper beam as claimed in claim 15, wherein the wallof the hollow body of each energy absorber is formed by a succession offacets.
 19. The bumper beam as claimed in claim 18, wherein the facetsare of identical width.
 20. The bumper beam as claimed in claim 18,wherein there are twenty-four facets.
 21. The bumper beam as claimed inclaim 18, wherein the facets make an angle of 135° between them.
 22. Thebumper beam as claimed in claim 1, wherein the hollow body of eachenergy absorber has a first end connected to the front sole plate and asecond end resting on a longeron of the structure of the motor vehicleand passing through the rear sole plate via an orifice which has anoutline of a shape that complements the cross section of said hollowbody.
 23. The bumper beam as claimed in claim 1, wherein the hollow bodyof each energy absorber has a first end resting on the front sole plateand a second end resting on a longeron of the structure of the motorvehicle and passing freely through the rear sole plate via an orificewhich has an outline of a shape that complements the cross section ofsaid hollow body, the hollow body being connected to the rear soleplate.
 24. The bumper beam as claimed in claim 1, wherein the hollowbody of each energy absorber has a first end resting on the front soleplate and a second end connected to the rear sole plate, said sole plateresting on a longeron of the structure of the motor vehicle.
 25. Thebumper beam as claimed in claim 1, wherein the hollow body of eachenergy absorber rests on the front sole plate and is connected to theend edge of said core.
 26. The bumper beam as claimed in claim 1,wherein the hollow body of each energy absorber rests on the front soleplate and is formed of two symmetric hollow half-bodies connected tosaid core and joined together at their free edges.
 27. The bumper beamas claimed in claim 1, wherein the hollow body of each energy absorberrests on the front sole plate and is formed of two symmetric hollowhalf-bodies each extending from a core, the two hollow half-bodies beingjoined together at their free edges.
 28. The bumper beam as claimed inclaim 1, wherein the metallic material of the hollow body of each energyabsorber has a thickness of less than 1.2 mm.
 29. The bumper beam asclaimed in claim 1, wherein the two sole plates and said core are joinedtogether.
 30. The bumper beam as claimed in claim 1, together with abumper synthetic foam coating.